New Regionally Adjusted Local Magnitude Models for Switzerland and Surrounding Regions

Summary

We present a new, regionally adjusted local magnitude (ML) model for Switzerland and surrounding regions. The model is derived based on Wood-Anderson displacement amplitudes (AWA) calculated from 150,000 high-quality waveforms from 15,000 earthquakes between 2000 and 2025, recorded by more than 700 seismic instruments. This dataset is substantially richer than those used in previous ML studies in Switzerland, with a large number of near-source recordings and data from low-magnitude events, which were notably sparse in earlier works. AWA attenuation over hypocentral distance is parametrised through linear and logarithmic distance terms along with hinge distance points, which allow proper modelling of the attenuation characteristics at long distances and changes in attenuation associated with post-critical reflected phases. Regional differences in attenuation between the Alpine region in southern Switzerland and the northern Foreland are smoothly modelled through a ray-path-specific regional adjustment parameter, allowing the model coefficients and the hinge distances to vary spatially. The coefficients of the parametric attenuation curves are estimated using mixed-effects regressions, and the model is anchored to yield a magnitude 3 for an AWA of 10 mm measured at a hypocentral distance of 17 km. The station terms are calculated with respect to Swiss reference rock conditions. The new ML model reduces uncertainty by 33 per cent compared to the current ML scale used by the Swiss Seismological Service and does not exhibit any residual trends with respect to hypocentral distance, earthquake depth, local site conditions, or event magnitude. Empirical radiation pattern corrections are derived, further reducing the uncertainty by 8 per cent for strike-slip events. Alternative models, based on non-parametric and cell-based 2D approaches, are derived independently to validate the parametrisation of the parametric model. The new model – MLS26 – yields lower magnitudes for smaller events (with catalogue magnitudes lower than about 2.5) and for events located in the northern Foreland, whereas the magnitudes of the larger Alpine events remain similar. The reduced magnitudes of smaller events decrease the b-value of the input earthquake catalogue from 1.00 to 0.93, corresponding to a reduction of about 7 per cent. MLS26 scales one-to-one with moment magnitude (MW) for MLS26 > 4, while for smaller events, it scales with the logarithm of the seismic moment.